The present invention relates to a traveling wire EDM apparatus and more particularly to a novel electrode wire guide and support structure for a traveling wire EDM apparatus.
It is known that traveling wire EDM apparatus utilize an electrode tool in the form of a metallic wire held under traction between a pair of support and guide members. The wire is continuously fed, at relatively high rate, in a longitudinal direction such as to continuously present a fresh portion of the wire in the machining zone between the electrode wire and the workpiece. The EDM apparatus is provided with a pulse generator connected across the electrode wire and the workpiece, and with a numerical control commanding a pair of servo-motors, the X-axis and Y-axis servo-motors, for displacing the longitudinal axis of the electrode wire relative to the workpiece according to a pre-programmed path such as to cut the workpiece, according to a predetermined shape, by the electrical discharges occurring in the machining zone between the the electrode wire surface and the workpiece surface.
Conventionally, the workpiece is held along a horizontal plane and the electrode wire is held vertically by the support and guide members, one support and guide member being disposed above the workpiece and the other being disposed below the workpiece. Each of the support and guide members is mounted on the end of a support arm. In some cutting operations, it is desirable to incline the longitudinal axis of the electrode wire relative to the workpiece at an angle other than a right angle. In order to accomplish this, the usual arrangement consists in mounting the upper electrode wire support and guide member on an intermediary movable member, rather than directly on the support arm, such as to provide the upper support and guide member with a limited amount of travel along one horizontal axis, or along two perpendicular horizontal axes, such as to displace the upper support and guide member away from its support and guide member. The mechanism permitting to incline the electrode wire may consist of a small cross-slide table, generally referred to as a UV table, displacing the wire support and guide member according to Cartesian coordinates or of an eccentric, generally referred to an R.crclbar., displacing the support and guide member according to polar coordinates. If it is desired to incline the electrode wire, in the course of a machining operation, to any practical angle and orient the wire in any direction, two additional servo-motors are required, one for each axis when using a cross-slide table, the axes being referred to as the U-axis and the V-axis.
In application Ser. No. 208,934, filed Nov. 21, 1980 and assigned to the same assignee as the present application, a structure is disclosed which consists of a single slide mounted on the upper support arm, and a single slide mounted on the lower support arm, both slides being displaceable in a horizontal plane, one along an axis perpendicular to the axis of displacement of the other slide.
Slide mechanisms present an important inconvenience. They must have many moving parts in order to convert rotating motions into linear motions. For example, the leadscrew-nut assembly introduces a certain amount of play and backlash into the mechanism, and the slide provided with the nut for the leadscrew is supported from ways which introduce another amount of play and backlash. In arrangements consisting of a cross-slide table, the superimposition of the motions along the U-axis and the V-axis doubles the amount of play and backlash and, of course, additional errors must be added, such errors resulting from the machining tolerances and the geometric imperfections of the bearing surfaces.
Attempts have been made at reducing such cumulative errors by eliminating the slide. For example, one proposed solution, as disclosed in published German patent application No. 2401424, consists in mounting the workpiece by way of universal joint, such as to be able to incline the wire in any direction relative to the workpiece. In published German patent No. 2644369, it is the whole support arm assembly which is made pivotable in all directions by means of bearing surfaces disposed on cylindrical surfaces. Such solutions eliminate therefore any linear motion, but they present the disadvantage of requiring a mechanism somewhat rigid and powerful for the electrode wire guide and support member in order to pivot with precision a heavy and cumbersome assembly. In addition, in heavy EDM apparatus, the present state of the art requires a gear reduction mechanism to be incorporated in the system between the servo-motors and the heavy movable assembly, which, in turn, requires a chain of moving parts which reintroduce all the play and backlash which were sought to be eliminated in the first place.